Many readers wrote to confirm the previously reported "-51" errors when using Disk Express Pro. Claudio Palma claims to have eliminated the problem by locating an invisible file (using a utility such as File Buddy) called "DiskExpress Activity Log" and deleting it.
Alsoft tech support wrote to Jorg Freiberg regarding another error number, but with the same basic solution: "Error -411 is generated because of a corruption in the DiskExpress Activity Log. The log must be deleted in order to correct this problem. Open DiskExpress and click the Disk Options tab. Disable the Monitor All checkbox, then select the disk that you were having problems in the Available Disks: list. Following this disable the Monitor Disk checkbox, this will bring up a dialog box warning you that the action you are about to take will delete all the information in the log. Choose Turn Off, once this step is completed enable the Monitor Disk checkbox again to create a new log that has no corruptions in it. This will solve your problem and you should not receive the -411 again."